Nitrogen deposition and forest nitrogen cycling along an urban-rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha⁻¹ yr⁻¹, while the throughfall covered a larger range of 11.7-65.1 kg N ha⁻¹ yr⁻¹. Foliar N concentration, NO₃ ⁻ leaching to stream, and soil NO₃ ⁻ concentration were low and NO₃ ⁻ production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃ ⁻ leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between −6.6‰ and 0.7‰, and was negatively correlated with soil NO₃ ⁻ concentration and NO₃ ⁻ leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N-depleted soil NO₃ ⁻, (2) foliage uptake of ¹⁵N-depleted NH₄ ⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.